During oil and gas extraction, the well is typically lined using casing which comprises a number of steel pipes, typically each having a length of around ten meters, which are connected together as they are run into the well then cemented in place. Prior to completion of the well, fluid is typically circulated to remove debris from the well. The drilling fluid is pumped down the internal bore of the drill string to the bit and returns in the annulus between the drill string and the well casing. However, this cleaning process can be inefficient for a number of reasons.
Step changes in the internal diameter of the borehole and casing can reduce the flow rate of the cleaning fluid. It is desirable to be able to direct cleaning fluid, preferably at a higher rate, at particular vertical sections of the casing. It is known to provide tools connected within the drill string which can selectively redirect fluid through transverse ports provided at the tools.
These known tools are typically moveable between an extended position in which the transverse ports are closed and a collapsed position in which the transverse ports are open. A mandrel of the tool slides within the outer body to block the transverse ports when the tool is in the extended position. Also, the tools may be adapted such that no torque is transmitted between the portion of the drill string above the cleaning tool and the portion of the drill string below the tool when the tool is at the collapsed position.
Movement of the known tool between the extended and collapsed positions can be performed in a number of ways. One method is to support a shoulder of the tool on the top of a liner and move the tool to the collapsed position by increasing the load on the top of the tool and compressing a spring.
Therefore, fluid can flow through the transverse ports at this location. Another method is to lift the drill string so that the weight of the drill string below the tool causes the tool to move to the extended position. The tool is moved to the collapsed position by lowering the drill string. Therefore, fluid can flow through the transverse ports when the drill string has been lowered.
It is desirable that the tool can be collapsed without using a portion of the casing or without having to lower the drill string so as to allow flow through the transverse ports at more vertical locations within the borehole and casing.
It is often necessary to operate valves located at a lower end of the drill string. A common method of doing this is to drop balls or darts down the internal bore of the drill string including the cleaning tool to contact and operate the valves. The cleaning tool provided within the drill string should not interfere with this process. However, known tools require step changes in the internal diameter of the through bore of the tools, since components such as the mandrel and outer body must slide relative to each other as the tool moves between the extended and collapsed positions. During normal operation, with the tool at the extended position, there is an internal dimensional step change for fluid flowing through the tool. A dropped ball which deviates from the central region of the bore of the outer body may fail to flow into the mandrel. Also, the non-laminar flow in the region of the step change may alter the flow path of the dropped ball which again may fail to flow into the mandrel. Even without step changes and non-laminar flow, the long run of the ball within a tool (which is rarely substantially vertical) means that the ball seldom remains at the centre of the tool.
Furthermore, a particular problem exists when deviated drilling is performed. In an angled well, the drill string is also at an angled orientation and, due to the effects of gravity, the dropped ball tends to travel to the low side of the tool and so may fail to flow into the mandrel for this reason.
It is desirable to provide means for ensuring that a dropped ball will consistently flow into the mandrel.